In providing materials for use in forging applications, it is known that materials formed from semi-solid thixotropic alloy slurries possess certain advantages, including improved part soundness. This results because the metal is partially solid as it enters the die cavity and, hence, less shrinkage occurs. Machine component life is also improved due to reduced erosion of dies and reduced thermal shock.
Methods for producing semi-solid thixotropic alloy slurries known in the prior art include mechanical stirring and inductive electromagnetic stirring. The process for producing such a slurry with the proper structure requires a balance between the shear rate imposed by the stirring and the solidification rate of the material being cast. The metal composition is characteristically either a solid or partially solid and partially liquid which comprises primary solid discrete particles in a secondary phase. The secondary phase is solid when the metal composition is solid and liquid when the metal composition is partially solid and partially liquid. The compositions are formed from a wide variety of metals or metal alloy compositions, while the primary particles comprise small degenerate dendrites or nodules which are generally spheroidal in shape and are formed as a result of agitating the metal alloy composition when the secondary phase is liquid. The primary solid particles are made up of a single phase or plurality of phases having an average composition different from the average composition of the surrounding matrix, which matrix can itself comprise primary and secondary phases upon further solidification.
Normally solidified alloys, in the absence of agitation, have branched dendrites separate from each other in the early stages of solidification, i.e., up to 15-20 weight percent solid, which develop into an interconnected network as the temperature is reduced and the weight fraction solids increase. Prior art, such as U.S. Pat. No. 3,954,455, teaches a method of preventing the formation of interconnected networks by maintaining the discrete primary particles separated from each other by the liquid matrix up to solids fractions of 60-65 weight percent or higher. The primary solids are degenerate dendrites in that they are characterized by having smoother surfaces, fewer branched structures, and a more spherical configuration as compared to normal dendritic structures.
There are several ways of forming alloy compositions useful in practicing the present invention which are all well known in the prior art. Typically, a metal alloy is first melted to a liquid state and introduced to a device which is capable of agitating the liquid during its solidification. The liquid-solid mixture can, when the desired ratio of liquid and solid has been reached, be cooled rapidly to form a solid slug for easy storage. Later, the slug can be raised to a temperature to form a liquid-solid mixture and then subjected to a casting or forging process to form the desired final part. The alloy thus possesses thixotropic properties when reheated to the liquid-solid state. In such a state it can be fed into a modified die casting or forging machine in apparently a solid form. However, shear resulting when this apparently solid slug is forced into the die cavity causes the slug to transform to a material whose properties are more nearly that of a liquid. An alloy slug having thixotropic properties can also be obtained by cooling the liquid-solid mixture to a temperature higher than that at which all of the liquid solidifies and the thixotropic composition can be cast or forged in that state.
The prior art has recognized that in preparing thixotropic alloy compositions, a surface skin tends to form on the preform ingot or slug as a result of an absence of agitation at the interface of the alloy composition and inner wall of the holding vessel. The prior art has attempted to reduce this problem by insulating the holding vessel during agitation and retard cooling of the alloy. Although the prior art has experienced various degrees of success in producing substantially uniform thixotropic compositions, it is virtually impossible to completely eliminate the dendritic "skin" from the finally-formed alloy ingot.
It is thus an object of the present invention to provide a process and apparatus for fabricating metal parts from thixotropic alloy compositions of the prior art which are substantially unaffected by the presence of the characteristic dendritic skin possessed by such thixotropic alloy ingots.
It is a further object of this invention to provide a process and apparatus for forming a forged metal part which is substantially stronger than corresponding forged metal parts of the prior art by producing the metal part from a thixotropic alloy composition substantially devoid of a surface containing dendritic skin and other skin-ladened impurities, which typically accompany thixotropic alloy slugs.